Dibenzo[d,g][1,3]dioxocin-6-carboxylic acids substituted by methyl or ethyl or a moiety --CH.sub.2 CH.sub.2 -- at the 12-position and optionally substituted at other positions, such as methyl 4'-chlorospiro(cyclopropane-1,12'(12'H)-dibenzo[d,g][1,3]dioxocin)-6'-carboxylic acid, and their agriculturally acceptable esters, amides, and salts are useful for the control of undesirable vegetation. The 1,1-diarylcyclopropane intermediates required for the spirocyclopropane compounds can be prepared from appropriately substituted 1,1-diarylethene procursors by reaction with phenylthiomethyl lithium reagent.